This invention relates generally to electrolytic techniques for treating and purifying contaminated aqueous solutions and remediating soil in situ by irrigating the soil and then treating the resulting contaminated aqueous solution electrolytically. As used herein, the term xe2x80x9ccontaminated aqueous solutionsxe2x80x9d refers to bulk aqueous solutions containing concentrations of colloidal particles, heavy metals, phosphate-containing species, micro-organisms, nitrogenous species, soluble organic matter, dissolved solids such as inorganic mineral salts, or any combination thereof.
Purifying Contaminated Aqueous Solutions
Present state-of-the-art techniques for treating and purifying contaminated aqueous solutions, such as sanitary wastewater, drinking water, groundwater and landfill leachate, primarily involve bio-oxidative techniques. Application of these bio-oxidative techniques is limited by serious problems. U.S. Pat. No. 5,853,588 to Molof, et al., notes that transient conditions generate a chemical imbalance which does not allow for adequate phosphate removal. U.S. Pat. No. 5,514,277 to Khudenko describes factors that complicate anaerobic digestion in standard biological treatment.
Conventional wastewater treatment, as presently practiced, typically involves three stages, namely, primary, secondary and tertiary treatment. Some form of sludge is usually generated at each stage. Sludge is mainly a material of bacterial origin formed at all three stages of conventional treatment but largely produced from the bio-oxidative (secondary) treatment stage. Sludge, a highly viscous concentrate of settled colloidal suspension with a mushy or mud texture, is the solids material which settles out during various stages of conventional wastewater treatment and which has to be processed and dewatered prior to being hauled off site for disposal.
Conventional Wastewater Treatment Stages
Primary treatment is the first stage of the process using mechanical methods to separate and remove sand, grit and larger solids from the influent (untreated or fresh wastewater). Screens, settling tanks and skimming devices are commonly used to effectuate the separation. The solid material which settles out in this process is generally referred to as xe2x80x9cprimary sludgexe2x80x9d.
Secondary treatment follows primary treatment and typically involves bio-oxidative techniques for reducing suspended, colloidal and dissolved organic matter in the effluent emanating out of primary treatment. Bio-oxidation, at best, produces sludge and usually performs less than ideally in practice due to a number of operational problems, such as toxic loading and temperature depressions. Activated sludge and trickling filters are two of the most common means of secondary treatment. In secondary treatment, wastewater typically passes through a series of holding and aeration tanks and ponds to further remove floating and settleable solids and about 90 percent of the oxygen-demanding substances and suspended solids. Disinfection, typically by chlorination, is the final step in secondary treatment.
Tertiary treatment encompasses the additional treatment of effluent beyond that of primary and secondary treatment methods, generally by chemical or physico-chemical means. It involves selected biological, physical and chemical separation processes to remove additional pollutants such as nitrogen and phosphorous. Examples of tertiary treatment include activated carbon treatment, removal of ammonia by anaerobic denitrification, removal of phosphates by liming, and germicidal treatment by ozonolysis, ultraviolet irradiation and chlorination. All such tertiary treatments are usually very expensive to operate and most often require the addition of chemical agents which always leave some unpleasant residue in the effluent.
Conventional tertiary treatment to remove ammonia cannot proceed by the direct transformation of ammonia to nitrogen gas, but must proceed by anaerobic denitrification whereby ammonia is oxidized to terminal nitrate ion and then anaerobically converted back to nitrogen gas.
A basic problem with anaerobic denitrification is that wastewater is typically poor in carbon compounds. Biological tertiary treatment to remove ammonia requires additional carbonaceous input to insure sufficient presence of carbon to support the bacteria that carry out the treatment. Such addition of carbon results in the further production of residual solids (often referred to as xe2x80x9ctertiary sludgexe2x80x9d). Phosphate removal accomplished by liming also produces additional tertiary sludge.
The concentrated solids residue remaining after secondary or tertiary treatment is further processed for reuse and/or disposal. Residual solids which purportedly meet certain health and safety criteria are called xe2x80x9cbiosolidsxe2x80x9d and can be recycled as fertilizer/soil conditioner, burned to produce energy, or made into other useful products. Residual solids which do not meet such criteria are called xe2x80x9csludgexe2x80x9d and must be hauled off site for disposal by means other than land application. The final solids by-product of conventional wastewater treatment systems falls within the category of sludge.
Problems with Conventional Wastewater Treatment
U.S. Pat. No. 5,837,142 to Mullerheim further describes the problems and complications associated with conventional wastewater treatment systems, which involve biological digestion of wastewater in the liquid phase. As indicated by Mullerheim, the digestion process is susceptible to disturbances of flow, nutrient loadings, temperature, chemical content, accumulated sludge levels and other influences. Digestion requires long retention times in large tanks. Close supervision of the process by skilled operators is often required for acceptable performance, although such supervision is no guarantee of a good outcome. In secondary treatment, organic nitrogenous wastes are not entirely removed by the processes, but rather, are converted into soluble nitrate compounds that could potentially pollute surface and ground waters.
The proper disposal of sludge has become a major problem by virtue of its ability to collect and retain heavy metals and toxic chemicals present in a waste stream, as well as its daunting physical properties and large water content. Incineration, landfilling, and ocean dumping all have major flaws and are strictly regulated. Due to the presence of bacteria in sludge, the impact of the disposal of sludge by land application is the likely invasion of the soil by heavy metals, toxic chemicals and pathogenic agents, in which case groundwater can be contaminated and can spread disease causing bacteria.
The problems and complications associated with conventional wastewater treatment systems are also encountered in the treatment and purification of other contaminated aqueous solutions.
Non-biological Processes
Products and processes developed for treating wastewater and other contaminated aqueous solutions by non-biological means have not found widespread use. Such approaches have involved electrolysis; the use of incineration; chemical treatment with coagulants, flocculants, adsorbants, filter aids and oxidants; radiation from nuclear sources; and physical treatments such as air flotation, filtration and centrifuging. Chemical and incineration approaches have been very expensive and energy intensive, often producing chemical laden sludges and air contaminants, which in themselves present a pollution problem. Filtration has been relatively unsuccessful because of the inability to achieve high rates of solids removal without fouling of the filters and frequent need for backwashes which in themselves create a disposal burden.
Electrolytic Removal of Contaminants
Various methods have been reported for electrolytically removing contaminants from aqueous solutions. U.S. Pat. No. 5,531,865 to Cole describes a method using an electrolytic apparatus wherein the electrodes include at least one elongate cathode and one or more elongate sacrificial floc-forming anodes aligned parallel with the cathode. The method described by Cole further includes the addition of chemical flocculating agents. In promoting both oxidation and flocculation simultaneously, without separating these two antagonistic processes, the effectiveness of both processes is substantially diminished.
The method of U.S. Pat. No. 5,531,865, as well as all other prior art electrolytic processes, such as U.S. Pat. No. 4,623,436 to Umehara and U.S. Pat. No. 3,933,606 to Harms, cannot achieve ammonia oxidation because of the presence of large amounts of organic material at the sacrificial anode.
Electrode Configuration
The operation and structure of the anode described in prior art references limits their effectiveness. For example, in U.S. Pat. No. 3,756,933, the outflow from the cathode area is merely subjected to bubbles of oxygen and chlorine from a non-sacrificial anode, without the ability to effectively utilize oxidative reactions. This limitation is due to the insufficiently intimate contact of the anolyte with the non-sacrificial anode surface. U.S. Pat. No. 4,948,489 suffers the same defect since the waste stream never contacts the anode, for a dummy anolyte is fed through the anode chamber. U.S. Pat. No. 3,756,933 uses dimensionally stable anodes of polished platinized materials, but fails to realize the benefit of having the current density on the anodes being at least on the order of 40 amps/ft2.
U.S. Pat. No. 4,179,347 to Krause describes a system for disinfecting and removing suspended solids from wastewater streams such as sewage and streams containing organic matter. The configuration of the electrolytic cell used by Krause employs parallel electrode plates, with solids removal being accomplished by means of skimming, suction and screening. In particular, Krause does not appear to appreciate the benefit of separation of anode and cathode chambers by an intermediate separation membrane. Krause explicitly acknowledges that only selected nutrient-containing solids are removed by his apparatus.
Chemical Additives
Additionally, the process described by Krause requires the addition of large concentrations of sodium chloride to the feed. The addition of sodium chloride to the feed to be treated ensures the production of an effluent containing levels of chlorine-containing organic residues, chloride ions and halomethanes, which clearly does not meet current EPA standards for secondary treatment. Moreover, such chemical addition also produces a solids residue which necessarily contains chlorides and chlorinated organic species, both prohibitive contaminants by present day standards.
The use of outside chemical agents which, at least in part, ultimately end up in the effluent (treated stream), is found in many prior art references to both electrolytic and non-electrolytic methods of purifying contaminated aqueous solutions. For example, U.S. Pat. No. 4,872,959 to Herbst, et al., refers to an electrolytic treatment method employing the addition of chemical substances to effect treatment; U.S. Pat. No. No. 4,208,283 to Brozes describes a method of sanitary wastewater treatment effected by raising the pH of the stream with lime to coagulate the dissolved and suspended solids, followed by separation of the solids, and finally by the addition of chlorinating agents to disinfect the partially treated stream.
Membrane Filtration
Mullerheim, in U.S. Pat. No. 5,837,142, describes a method and apparatus for treating sanitary wastewater utilizing a membrane filtration system which separates wastewater into liquid and concentrated solids components by means of membrane filtration. Vibratory shear methods are employed to minimize fouling or blinding of the filtration medium. The resulting solids component is thereafter dried, disinfected and deodorized by a variety of methods to facilitate storage and/or disposal. The permeate produced by Mullerheim""s process contains both micro-organisms and heavy metals, requiring elaborate, environmentally-sensitive and costly additive measures for purposes of disinfecting and deodorizing the permeate and chemically oxidizing putrescible compounds contained therein. The precipitate produced by Mullerheim""s process is a sludge requiring elaborate and costly post-filtration treatment methods such as composting with wood processing and cement production wastes and the end result of Mullerheim""s process is still sludge.
In Situ Remediation of Soil
Many different techniques have been proposed for removing contaminants from soil, all of which suffer from one or more disadvantages that have made their use either technically or economically impractical.
Some remediation techniques, such as soil washing and incineration, require the excavation and subsequent off-site treatment of the contaminated soil and are therefore unsuitable for large-scale treatment because of the immense costs associated with digging and heating. Additionally, each of these methods pose significant health hazards to workers and to the environment. In the case of incineration, a site pollution problem is oftentimes replaced with an air pollution problem.
Proposed methods for in situ soil remediation include bioremediation, injection techniques, and electrokinetics, all of which produce some form of biosolids and/or aqueous waste as a by-product, which must be further processed for reuse and/or disposal.
Bioremediation involves a biodigestive process generally utilizing bacteria and fungi. Biodigestion is dependant upon an adequate supply of heat, aeration, water and nutrients. Inadequacies in the supply of one or more of these elements will impede and eventually stop the decomposition process. Additional operational and environmental problems are posed by the difficulty in confining microbial activity within a given region and secondary contamination resulting from the inability to recover degrading bacteria once proliferated in the soil.
Techniques for injecting chemical or biological agents into soil are generally restricted to soils having relatively high hydraulic permeability, i e., relatively sandy soils, and further suffer from the difficulty of achieving a uniform distribution of the detoxifying agent(s) throughout the soil.
Electrokinetic processes generally involve the migration of contaminant-containing water through the soil under the influence of an electrical field. The water in the soil is caused to migrate toward and accumulate at or near one or more electrodes, the accumulated water therein being removed by pumps. The accumulated water must undergo extensive further treatment to reduce or eliminate the levels of contaminants contained therein.
Presently proposed in situ remediation methods, including high pressure soil flushing, vacuum or steam extraction, or radio frequency volatilization, are unable to remove some of the trace level contaminants of greatest environmental concern, such as toxic heavy metals, whose strong attachment forces bind them to the soil particles.
The ideal technique for treating and purifying contaminated aqueous solutions would be an economical, non-biological process in which all the contaminants of greatest environmental concern are either removed entirely or reduced to environmentally acceptable limits without the use of chemical additives or biologically active materials and without producing sludge as a by-product of treatment. Despite the major environmental and economic concerns associated with the handling and disposal of sludge, to date, no method of treating such contaminated aqueous solutions has been developed which accomplishes the aforementioned objectives. An object of the present invention is to provide such a method.
This invention relates to the electrolytic treatment of contaminated aqueous solutions, including diffusate or leachate extracted from soil, and to a novel means of purifying such aqueous solutions without the need for chemical additives or biologically active organisms. The invention avoids creating malodorous effects or sludge as a by-product of treatment. Agglomeration of the colloidal matter in contaminated aqueous solutions is achieved by the use of the electrolytic unit of the present invention and the presence of sufficient cations within the feed solution to be treated.
The genesis of this invention was the realization that most natural contaminants in contaminated aqueous solutions are negatively charged colloidal particles, that neutral colloidal particles in such aqueous solutions can be altered so as to acquire a negative charge, and that a system designed to exploit these negative charge characteristics would satisfy the long felt need for a decontamination and remediation system that avoids sludge production and the use of chemical additives or biologically active materials.
The present invention rests in part upon the discovery that the surface layer of colloidal particles, whether hydrophobic, hydrophilic, or a combination of both, can be permanently altered in such a way as to cause total and irreversible colloidal destabilization (agglomeration) of all the colloidal material present in an aqueous solution without the use of chemical additives. This novel phenomenon is apparently accomplished as a result of some, if not all of the chemical and physico-chemical reactions that the present invention causes to occur at the cathode-solution interface of an electrolytic cell.
The invention, embodied in a treatment unit apparatus, involves several unique concepts. The invention employs a unique electrolytic cell and cathode configuration to bring about the agglomeration of colloidal particles in the contaminated aqueous solution within a cathode chamber. It does this by producing hydroxyl ions from the electrolysis of water and by incorporating positive charge centers throughout the surface of the grossly negative cathode. The preferential adsorption of the hydroxyl ions to the material surface of all the colloidal particles in the aqueous solution ensures that all such colloidal particles are electronegative in nature. The positive charge centers on the cathode surface drive the migration of negatively charged colloidal particles into the cathode-solution interface along with cationic matter which is migrated therein by normal electrolytic transference. The co-concentration achieved by these parallel processes causes the compression (collapse) of the Gouy-Chapman layer about the colloidal particles and their agglomeration within the cathode-solution interfacial zone.
The configuration of the cathode (a V shaped configuration), within a cathode chamber, assures that hydrogen bubbles evolved from the electrolysis of water will move colloidal particles in a steady state stream within the cathode-solution interface. The positive charge centers on the cathode surface attract negatively charged colloidal particles into the cathode-solution interface so that a steady agglomeration process continuously occurs. The spent catholyte is then fluidly transported from the treatment unit""s electrolytic cell to a filter chamber where solids are removed.
A feature of the invention is the separation of the cathode chamber from an anode chamber by a membrane of submicron porosity. This physically separates the agglomeration process from oxidation processes within the anode chamber of the electrolytic cell and restricts osmotic reflex (from anode to cathode chamber). Agglomeration and oxidation are contrary processes, one involving the building up of larger structures, the other the breaking down of structures, and their separation in this invention is particularly advantageous.
Another feature of the invention is the maintenance of a high pH at the cathode, which enables the total and irreversible agglomeration of all the colloidal particles to occur at the cathode-solution interfacial zone and which simultaneously enables the precipitation of phosphates, the denaturing of microbiological material and the hydrolysis of urea to ammonia.
In lieu of changing the electrical potential applied to the electrodes of the electrolytic cell, the treatment unit may include mechanical means for adjusting the flow rate of the feed solution through the cathode chamber of the electrolytic cell in order to maintain the pH level of the cathode chamber in a predetermined range.
Still another feature of the invention is the maintenance of a high current density on the anode, which facilitates the oxidation of ammonia to nitrogen gas and also produces chloric acid in the anode chamber to oxidize any residual soluble organic material and to act germicidally.
Additionally, the invention provides a fluid flow path from the cathode chamber through a filtration system with a return to the anode chamber, enabling the separation of the solid content of the spent catholyte, which includes, as a result of this process, the colloidal particles removed from the infeed.
The invention also contemplates an injection unit for adding cations in the feed in order to insure that the co-concentration of the colloidal particles and cationic matter in the cathode-solution interface is of sufficient strength to cause the collapse of the Gouy-Chapman layer about the colloidal particles.
The invention provides an electrolytic treatment unit that includes an electrolytic cell having a cathode chamber and an anode chamber separated from the cathode chamber by a separation membrane of sub-micron porosity, an electrical circuit for providing an electrical current through the electrolytic cell by applying a direct electrical potential across the electrodes of the electrolytic cell, a spent catholyte holding tank fluidly connected to receive catholyte from the cathode chamber, a filter chamber fluidly connected downstream from the spent catholyte holding tank, an anode feed holding tank fluidly connected downstream from the filter chamber and a flow control valve to stabilize the flow from the anode feed holding tank to the anode chamber. The filter chamber contains a membrane filtration medium which separates a clarified liquid collection zone from a precipitate collection zone in the filter chamber. Of course, the filter chamber need not be a simple structure, but may have its various components distributed among fluidly connected components. A pump directs the flow of spent catholyte slurry (unfiltered liquid) from the spent catholyte holding tank to the filter chamber and through the membrane filtration medium at a predetermined pressure or rate. The clarified liquid collection zone in the filter chamber is fluidly connected to an anode feed holding tank. Anolyte from the anode feed holding tank is fed to the anode chamber of the electrolytic cell preferably by gravity flow through a flow control valve. Precipitate from the precipitate collection zone of the filter chamber is removed and collected in a solids holding tank, after which it can be further processed to extract heavy metals, if desired.
The treatment unit preferably also includes a unique method for back-pulsing liquid through the membrane filtration medium to unbind the filter material that requires only one valve and no compressed air assist, in contrast with known back-pulsing methods. Means may also be provided for injecting agents, such as mineral acid, into the filter chamber for cleaning the membrane filtration medium of blocking compounds, such as calcium carbonate or ferric hydroxide.
To deal simultaneously with the phosphate contaminants, the electrical current that electrolyzes the water molecules produces hydroxyl ions in the cathode-solution interfacial zone, thereby raising the pH. This high pH further converts phosphate ions into orthophosphate resulting in the precipitation of phosphate as alkaline earth phosphate.
The hydroxyl ions produced from the electrolysis of water serve an additional significant purpose. In the case of colloidal particles in aqueous solutions, whether hydrophobic or hydrophillic, the preferential adsorption of hydroxyl ions to the material surface of all such colloidal particles ensures that all such colloidal particles, even neutral colloidal particles, will acquire a negative charge.
The chemical transformations occurring simultaneously at the cathode-solution interface further result in the denaturing of micro-organisms and the removal of dissolved inorganic solutes in the form of water hardness from the feed, thus lowering the Total Dissolved Solids [xe2x80x9cTDSxe2x80x9d]. This is in direct contrast to conventional treatment methods, which do not denature and which raise the level of TDS in order to precipitate phosphate. The treatment unit also includes apparatus for measuring and controlling the pH level in the cathode chamber.
These are the main features of the apparatus and process. Others are described in the detailed description of the invention.
Accordingly, it is an object of this invention to provide a process and apparatus for treating and purifying contaminated aqueous solutions containing concentrations of colloidal particles by a non-biological process which utilizes one or more electrolytic cells designed to agglomerate the colloidal particles within an interfacial zone adjacent a cathode and does not require the use of chemical additives or flocculants.
It is a further object of this invention to provide a process and apparatus for treating and purifying contaminated aqueous solutions in an electrolytic cell wherein a cathode chamber and at least one anode chamber are separated by a separation membrane whose structure allows conductivity driven ionic transfers, contains the colloidal particles within the cathode chamber so that they can be agglomerated and subsequently harvested by filtration, and restricts osmotic reflux.
It is a further object of this invention to provide a configuration for the cathodes that contributes to a steady state flow of colloidal particles and cations in the cathode-solution interface within the electrolytic cell sufficient to collapse the Gouy-Chapman layer about the colloidal particles, thereby bringing about total and irreversible agglomeration of the colloidal particles.
It is a further object of this invention to provide a means for increasing the quantity of cations in the aqueous feed solution in order to insure that the co-concentration of the colloidal particles and cationic matter in the cathode-solution interface causes the collapse of the Gouy-Chapman layer about the colloidal particles.
It is a further object of this invention to provide an electrolytic treatment system wherein a stable flow (moderated if necessary by a flow control valve or other such fluid control means) of fluid through the system provides that the agglomerated colloidal particles are removed by filtration from the catholyte.
It is a further object of this invention to provide for the electrolysis of water in such a treatment system and the production of hydroxyl ions at the cathode of an electrolytic cell whereby the preferential adsorption of hydroxyl ions to the material surface of the colloidal particles contained in the contaminated aqueous solution ensures that all such colloidal particles, including neutral colloidal particles, will acquire a negative charge.
It is a further object of this invention to provide for the electrolysis of water in such a treatment system and the generation of hydrogen bubbles in such a form as to drive the steady state non-turbulent circulation of colloidal particles in the cathode solution interface and to raise the transient pH in the cathode solution interface above 12 in order to bring about the precipitation of phosphates as alkaline earth phosphates and the denaturing of micro-organisms and any other material of biological origin.
It is a further object of this invention to provide a means of using electrolytic cells for the de-solubilizing of phosphates and the denaturing of micro-organisms and any other material of biological origin.
It is a further object of this invention to provide an electrolytic treatment system that oxidizes ammonia to nitrogen gas and also produces chloric acid in the anode chamber to oxidize any residual soluble organic material and to act germicidally.
It is a further object of this invention to provide a means of using electrolytic cells for the hydrolysis of urea to ammonia and the oxidation of ammonia to nitrogen gas.
It is yet a further object of this invention to provide a method for using the electrolytic treatment system of the present invention to remediate soil contaminated with sludge and/or environmental pollutants.
It is yet a further object of this invention to provide a method for using the electrolytic treatment system of the present invention to treat and purify diffusate or leachate extracted from soil that is contaminated with concentrations of colloidal particles, heavy metals, phosphate-containing species, micro-organisms, nitrogenous species, soluble organic matter, dissolved solids such as inorganic mineral salts, or any combination thereof.
It is a further object of the invention to provide a final effluent from the anode chamber that is entirely free of micro-organisms (and any other material of biological origin) and heavy metals, has low enough concentrations of organic carbon, nitrogen, phosphorous and TSS to allow for its direct discharge to the environment in compliance with current applicable discharge regulations, and is non-infectious and nonmalodorous.
These and other objects of the invention will become more apparent from the description of a detailed embodiment below.